This proposal focuses on the application of pericyclic reactions in new methodology for the synthesis of several important classes of organic compounds. General, stereocontrolled routes to functionalized cyclopropane derivatives will be developed based on the chemistry of cyclopropylboranes. Anion-accelered [1,3] sigmatropic rearrangements will be employed in stereocontrolled annulation approaches to five-membered carbocyclic compounds. A regiocontrolled annulation route to highly substituted aromatic compounds will be developed and applied to the total synthesis of the biologically important natural products bhogatin, ascochlorin, cristatin, barbatusol, and mycophenolic acid